Polarimeters are measurement instruments used to determine the optical rotation of liquid samples. A typical arrangement includes a first optical system to project a beam of light of known polarization state through a liquid sample. The beam exiting the sample then passes into a second optical system which detects a change in polarization state induced by the liquid sample. The means of holding the liquid sample is commonly referred to as a sample cell. Sample cells often have a cylindrical inner chamber arranged coaxially with the beam. Each end of the cylindrical chamber is closed off with a transparent window to confine the liquid sample while allowing the beam to traverse the cylindrical chamber. Filling ports are typically provided near each end of the sample cell. These ports communicate with the sample chamber and allow a liquid sample to be introduced into the cylindrical chamber from one end while air displaced by the incoming liquid sample exits through the port at the opposite end.
The property being measured, optical rotation, is proportional to the length of the beam path through the sample. A cylindrical chamber with a diameter slightly larger than the beam maximizes the length of the beam path for a given volume of liquid sample, thereby maximizing the detected signal.
A disadvantage of this arrangement is that any bubbles trapped in the sample chamber may also lie in the path of the beam of light. Bubbles may be present in the liquid sample prior to injection or may be created during injection by cavitation or a momentary loss of sealing between the filling port and the injecting device. Bubbles are a source of error in measurement of the optical rotation of liquid samples and should be detected and eliminated after filling to ensure correct results. Bubbles can be eliminated by emptying and reloading the sample cell or by pushing additional liquid sample through the sample cell.
The traditional method of checking that a sample cell has been filled without bubbles is to raise the sample cell to eye level and sight along the bore of the sample chamber through the transparent windows. A more recent development is to place an electronic camera along with suitable optics inside the polarimeter to replace the action of the eye.
Both the prior art built-in camera and traditional eye methods are incompatible in some ways with modern laboratory safety practice. Modern safety practice dictates that many materials should be handled under special conditions, for example under a fume hood. Typically a central sample preparation area is provided with suitable sinks, ventilation and other safety equipment. Sample cells are filled in the preparation area, often capped, and then delivered to the various measurement instruments.
The built-in camera method delays the detection of bubbles until the samples arrive at the instrument where conditions are not suitable for elimination of bubbles. The safety equipment is absent and any spilled materials may contaminate or damage the instrument. Sample cells must shuttle between the instrument and preparation areas until a proper fill is obtained.
The eye method can be carried out in the preparation area remote from the instrument, but the filling ports of the sample cell should be securely capped before bringing possibly hazardous materials near the face and eyes. A laboratory worker may spill the contents of an uncapped sample cell as they incline the cell toward a convenient source of light.
Therefore, it would be desirable to provide a way to safely verify that sample cells are properly filled in the preparation area away from a polarimeter, preferably at work bench height or while reaching into a fume hood.